fix: escape filter when input is undefined

src/builtin/filters/html.ts

@@ -14,7 +14,7 @@ const unescapeMap = {
 }
 
 function escape (str: string) {
-  return String(str).replace(/&|<|>|"|'/g, m => escapeMap[m])
+  return !!str ? String(str).replace(/&|<|>|"|'/g, m => escapeMap[m]) : ""
 }
 
 function unescape (str: string) {

test/integration/builtin/filters/html.ts

@@ -12,6 +12,9 @@ describe('filters/html', function () {
     it('should escape function', function () {
       return test('{{ func | escape }}', 'function () { }')
     })
+    it('should escape undefined', function () {
+      return test('{{ nonExistent.value | escape }}', '')
+    })
   })
   describe('escape_once', function () {
     it('should do escape', () =>